This invention relates to electrolytic systems and more particularly to electrolytic cell design.
Several types of electrolytic processes and cell designs are well known in many industrial branches of electrochemistry. "Diaphragm" type cells are often used in the electrolysis of aqueous solutions. Diaphragm cells generally have anolyte and catholyte compartments which are separated by an electrolyte separator such as a diaphragm or membrane. The compartments contain an anode and a cathode, respectively. Electrolyte separators prevent interaction of the anolyte and catholyte and separate the gaseous products evolving from each cathode. In the chlor-alkali industry, for example, cells having electrolyte separators are widely used in the electrolysis of sodium chloride brine to produce sodium hydroxide and chlorine gas.
Electrolyte separators may be of the rigid type, such as porous sheets of plastic or ceramic. Plastic or ceramic separators, when saturated with electrolyte, are usually nonselectively conductive to anions and cations.
Separators may also be of the flexible permeable type, such as woven textiles, nonwoven fibrous mattes or ion exchange membranes. Ion exchange membranes can be selectively conductive to either cations or anions.
Flexible separators, such as ion exchange membranes, have several advantages. These separators are not subject to fracture as are rigid separators. Furthermore, because they are generally thin, they are easy to fabricate and have minimal electrical resistance. As the cost of power to operate electrolyte cells increases, the need for these thin, flexible separators becomes more pronounced.
However, a major disadvantage to flexible separators is the swelling that usually occurs upon saturation of a separator with an electrolyte. The degree of swelling is dependent upon the concentration and the temperature of the electrolyte. This swelling can create slack in the separator.
Separator distortion is more likely to occur in a slackened separator. Most electrolytic cells employing separators have pressure differentials between the anolyte and the catholyte. Pressure differentials may be caused by density differences, hydrostatic head or kinetic head differences, etc., between anolyte and catholyte. These forces can cause separator distortion in the form of buckling, weaving or folding. This is particularly true of cells having vertically extending separators. Distortion may lead to a number of malfunctions in the electrolytic cell.
Distortion sometimes is in the form of separator fluttering. This may cause the separator to fail by fatigue at its points of support. Distortion of the separator can block the rise of the product gases and can cause an uneven flow of electrolyte past the electrodes. The distortion may also cause the separator to come into direct contact with an electrode, thereby slowing the electrolysis process at such areas of contact and increasing the current density in other areas. Excessive current densities can result in high "i.sup.2 r" drops and overheating.
Presently, there are no adequate means for eliminating separator swelling in flexible separators. In an attempt to eliminate the aforementioned effects of separator swelling, methods of taking up the slack created by the swelling have been devised. For example, in some cells the separator is stretched over a rigid frame and tension is applied by wedges inserted along one or more sides of the frame. As the separator swells, the wedges are pounded into the frame, thereby taking up the slack in the separator.
It is common practice in using some electrolytic cells to attempt to maintain the separator in a flat or planar configuration across the entire width and height of the cell. In an attempt to maintain this configuration, separators have been secured and tensioned along their peripheries. However, the aforementioned distortion problems often occur with this separator configuration. When pressure differentials exist on either side of the separator it can require an extremely high tension applied at the peripheries of the separator to pull the separator into a substantially planar position, especially for large separators which will have a larger total surface area. Further complicating the situation is the yield strength of the membrane which cannot be exceeded in any tensioning process. Ion exchange membranes, for example, are relatively thin and have relatively low yield strengths. This may limit the amount of tension that can be applied regardless of the tensioning method used.
One proposed method of coping with the aforementioned distortion problems of flexible separators is supporting a separator in a planar configuration between screens of nonconducting, structurally strong material. However, screens can create problems by blocking the movement of electrolyte and product gas. They can also increase the distance required between electrodes, thereby causing a higher resistance to current flow.
Reinforcing the separator internally with fibers or woven textiles has also been suggested. This may decrease separator swelling but it does not completely eliminate the problem in cells with large separators. Furthermore, non-conductive fibers added to the separator create additional power losses because of the increased difficulty of ion migration.
Separator distortion can also be reduced by the addition of intermediate separator supports in contact with the separator, as, for example, closely spaced nonconducting parallel bars attached to the electrode or the cell frame. Intermediate separator supports can also be in the form of ridges or lands on an electrode surface. The separator can be clamped between the ridges of opposing electrodes. For a vertically extending separator, these ridges usually are also vertical to permit electrolyte and product circulation upwards between the ridges.
Previously used intermediate separator supports, along with other aforementioned attempts at reducing separator distortion, have generally been aimed at maintaining the separator in flat or planar position entirely across the width of the cell. Attempts have been made along this line because it is generally believed that minimum power losses and maximum cell efficiency can be achieved with a flat or planar separator configuration.